The effect of wind speed on the presence of HEPA Filter

Air velocity is one of the most important factors in the effectiveness of an Air Filter. In daily use, we would say that the lower the air velocity, the better the filtration! The following is a brief analysis of this concept.
         The diffusion of small particle size dust (Brownian motion) is obvious, the air velocity is low, the airflow in the filter material is retained for a longer time, the dust will have more chances to hit the obstacle, therefore the filtration efficiency is high. Experience shows that for high efficiency filters, when the air speed is reduced by half, the dust transmission rate will decrease by nearly an order of magnitude (the efficiency value will increase by one 9), the air speed will double, and the transmission rate will increase by one order of magnitude (the efficiency will decrease by one 9).
         Similar to the effect of diffusion, when the filter material is electrostatic (electret material), the longer the dust is present in the filter material, the more likely it is to be adsorbed by the material. As the air speed changes, the filtration efficiency of the electrostatic material changes dramatically. If you know that electrostatic charges are present in the material, you should design your air conditioning system to minimize the airflow through each filter.
 
         According to conventional theory, when the air velocity decreases, the probability of collision between dust and fibers will decrease and the filtration efficiency will be lower. However, in practice, this effect is not obvious because the wind speed is small and the fibers are less elastic to dust, and dust is more likely to adhere.
         When the air velocity is high, the resistance is high. If the service life of the filter is based on the final resistance, the service life of the filter will be shortened when the air speed is high. It is difficult for the average user to observe the effect of air speed on filtration efficiency, but it is easier to observe the effect of air speed on resistance.
         For HEPA Filters, the air velocity through the filter material is typically 0.01-0.04 m / s. Within this range, the resistance of the filter is proportional to the air flow. For example, the initial resistance of a 484 * 484 * 220 mm high efficiency filter is 250 Pa at a rated air volume of 1000 m3 / h. If the actual air volume used is 500 m3 / h, the initial resistance can be reduced to 125 Pa. For a general ventilation filter in an air conditioning box, the air velocity through the filter material is in the range of 0.13-1.0 m / s. The resistance is no longer linear in relation to the air volume, but arcs upwards. A 30 % increase in air volume can increase the resistance by 50 %. If the resistance of the filter is a very important parameter for you, ask the air filter supplier for the resistance curve